1. Field of the Invention
The present invention relates to a method of operation of bipolar direct-current transmission system.
2. The Prior Art
Such a transmission system usually comprises two bipolar static convertor stations the center taps of which are earthed and which are connected to each other by means of two transmission lines, namely a positive and a negative conductor. The invention relates to such a transmission where both conductors are used also in single-pole operation, that is, after disconnection of one pole of the stations. In this way an earth-return conduction is avoided. The invention also relates to the direct-current transmission system itself.
As long as a bipolar direct-current transmission is working in normal operation, the two conductors conduct substantially the same current and the ground connections of the stations are substantially currentless or conduct relatively low currents for a relatively short time. These ground connections therefore do not have to be designed for a current load of any great magnitude or for a long period during normal operation.
However, if one pole of the transmission is cut off, the second one has to have some form of return conductor, and it may be inconvenient then to have to dimension the ground connections for this, and it can also be impermissible considering the resulting earth currents. For this reason, the plants are often planned so that, in single-pole operation, the free conductor is used as the return conductor. The simplest way to do this is to provide each pole in the stations with by-pass members, and in this case it is possible to use the normal by-pass members of the individual convertors as the return conductors.
When the free conductor is connected as a return conductor, the ground connection in one station must be broken since, particularly, in the case of long overhead lines, the conductor resistance is much higher than the ground resistance. In the first place, therefore, the transmission current will pass through the ground connection and, in order to disconnect this connection in a simple manner, it should first be made practically currentless. Previously this has been done by reducing the current to zero in the convertor of the remaining pole, which however causes a temporary interruption in the whole transmission.